The development of more efficient engines and power plants for future supersonic transports depend on the advancement of new high-temperature materials with temperature capabilities exceeding those of Ni-based superalloys. Having theoretical modelling techniques to aid in the design of these alloys would greatly facilitate this development. The present paper discusses a successful attempt to correlate theoretical predictions of alloy properties with experimental confirmation for ternary NiAl-Ti alloys. The B.F.S. (Bozzolo- Ferrante-Smith) method for alloys is used to predict the solubility limit and site preference energies for Ti additions of 1 to 25 at. % to NiAl. The results show the solubility limit to be around 5 % Ti, above which the formation of Heusler precipitates is favored. These results were confirmed by transmission electron microscopy performed on a series of NiAl-Ti alloys.

Tribology, the science and engineering of solid surfaces in moving contact, is a field that encompasses many disciplines: solid state physics, chemistry, materials science, and mechanical engineering. In spite of the practical importance and maturity of the field, the fundamental understanding of basic phenomena has only recently been attacked. An attempt to define some of these problems and indicate some profitable directions for future research is presented. There are three broad classifications: (1) fluid properties (compression, rheology, additives and particulates); (2) material properties of the solids (deformation, defect formation and energy loss mechanisms); and (3) interfacial properties (adhesion, friction chemical reactions, and boundary films). Research in the categories has traditionally been approached by considering macroscopic material properties. Recent activity has shown that some issues can be approached at the atomic level: the atoms in the materials can be manipulated both experimentally and theoretically, and can produce results related to macroscopic phenomena.

Adhesion, the binding of different materials at an interface, is of general interest to many branches of technology, e.g., micro-electronics, tribology, manufacturing, construction, etc. However, there is a lack of fundamental understanding of such diverse interfaces. In addition, experimental techniques generally have practical objectives, such as the achievement of sufficient strength to sustain mechanical or thermal effects and/or have the proper electronic properties. In addition, the theoretical description of binding at interfaces is quite limited, and a proper data base for such theoretical analysis does not exist. This presentation will review both experimental and theoretical aspects of adhesion in nonpolymer materials. The objective will be to delineate the critical parameters needed, governing adhesion testing along with an outline of testing objectives. A distinction will be made between practical and fundamental objectives. Examples will be given where interfacial bonding may govern experimental consideration. The present status of theory will be presented along with some recommendations for future progress and needs.

Rose, Smith, and Ferrante have discovered scaling relations which map the adhesive binding energy calculated by Ferrante and Smith onto a single universal binding energy curve. These binding energies are calculated for all combinations of Al(111), Zn(0001), Mg(0001), and Na(110) in contact. The scaling involves normalizing the energy by the maximum binding energy and normalizing distances by a suitable combination of Thomas-Fermi screening lengths. Rose et al. have also found that the calculated cohesive energies of K, Ba, Cu, Mo, and Sm scale by similar simple relations, suggesting the universal relation may be more general than for the simple free-electron metals for which it was derived. In addition, the scaling length was defined more generally in order to relate it to measurable physical properties. Further this universality can be extended to chemisorption and molecular binding. The implications of this scaling have been explored and have produced some interesting results and verifications. A simple and yet quite accurate prediction of a zero temperature equation of state (volume as a function of pressure for metals and alloys) is presented. Thermal expansion coefficients and melting temperatures are predicted by simple, analytic expressions, and results compare favorably with experiment for a broad range of metals. All of these predictions are made possible by the discovery of universality in binding energy relations for metals. Finally some results of other researchers concerning universality In Van der Waals forces are referred to.

We present experimental results on dry friction, which are consistent with the hypothesis that the stick-slip mechanism for energy release is described by self-organized criticality. The data, obtained with an Atomic Force Microscope set to measure lateral forces– examines the variation of the friction force as a function of time – or sliding distance. The materials studied were nominally flat surfaces of mica, quartz, silica and steel. An analysis of the data shows that the probability distribution of slip sizes follows a power law. Our data strongly supports the existence of self-organized criticality for nano-stick-slip in dry sliding friction.

Equivalent crystal theory (ECT) is a semi-empirical technique used for the calculation of defect energetics in metals and semiconductors. The implementation of the method involves the solution of transcendental equations. Although this is not a problem for simple defects, for complex defects, Monte Carlo and molecular dynamics calculations in large systems, it could be the speed-determining limitation in a calculation. In this paper we propose a procedure for bypassing this step and obtaining the desired result directly. The form of the particular transcendental equations suggest a different approach, namely, the equation can be cast in the form of the Lambert function that can be readily evaluated from standard routines. We test this scheme by evaluating the surface energies for a variety of metallic elements and by standard numerical approaches, and demonstrate that they agree to within a few ppm.

This paper presents an extension of the formalism of equivalent crystal theory (ECT) by introducing an electron density gradient term so that the total model density becomes a more accurate representation of the real local density. Specifically, we allow for the electron density around a lattice site to have directionality, in addition to an average value, as assumed in ECT. We propose that an atom senses its neighbouring density as a weighted sum—the weights given by the its own electronic probability. As a benchmark, the method is used to compute vacancy migration energy curves of iron. These energies are in good agreement with previously published results.

Experimental evidence and theoretical models are presented supporting the conjecture that dry friction stick-slip is described by self-organized criticality. We use the data, obtained with a pin-on-disk tribometer set to measure lateral force, to examine the variation of the friction force as a function of time. We study nominally flat surfaces of matching aluminum and steel. The probability distribution of force drops follows a negative power law with exponents mu in the range 3.2-3.5. The frequency power spectrum follows a 1/f alpha pattern with alpha in the range 1-1.8. We first compare these experimental results with the well-known Robin Hood model of self-organized criticality. We find good agreement between theory and experiment for the force-drop distribution but not for the power spectrum. We explain this on a physical basis and propose a model which takes explicitly into account the stiffness and inertia of the tribometer. Specifically, we numerically solve the equation of motion of a block on a friction surface pulled by a spring and show that for certain spring constants the motion is characterized by the same power law spectrum as in experiments. We propose a physical picture relating the fluctuations of the force drops to the microscopic geometry of the surface.

During the last 2 years, there has been a breakthrough in alloy design at the NASA Lewis Research Center. A new semi-empirical theoretical technique for alloys, the BFS Theory (Bozzolo, Ferrante, and Smith), has been used to design alloys on a computer. BFS was used, along with Monte Carlo techniques, to predict the phases of ternary alloys of NiAl with Ti or Cr additions. High concentrations of each additive were used to demonstrate the resulting structures.

The original equivalent crystal theory is a semi-empirical method for calculating the configurational energy of atomic systems. Each atomic site in the real crystal with defects is assigned an equivalent lattice constant, in general different from the ground state value. This parameter corresponds to a local compression or expansion of the perfect lattice. The basic method considers these volumetric transformations and, in addition, introduces the possibility that the reference lattice is anisotropically distorted. These distortions however, were introduced ad-hoc. In this work, we generalize the original Equivalent Crystal Theory by introducing site-dependent directional distortions of the lattice which account for the dependence of the energy on anisotropic local density variations. This is done in the spirit of the original framework of ECT, but includes a gradient in the density. This approach is introduced to ECT and may apply to other semi-empirical methods by making use of readily available first-principles results to fix parameters. We develop here the basic framework, and apply it to the calculation of Fe(110) and Fe(111) surface energy formation, and to the vacancy energy curve. The results, compared with first-principles calculations, show an improvement over previous semi-empirical approaches.

This letter presents experimental results on unlubricated friction, which suggests that stick–slip is described by self-organized criticality (SOC). The data, obtained with a pin-on-disc tribometer examines the variation of the friction force as a function of time—or sliding distance. This is the first time that standard tribological equipment has been used to examine the possibility of SOC. The materials were matching pins and discs of aluminium loaded with 250, 500 and 1000 g masses, and matching M50 steel couples loaded with a 1000 g mass. An analysis of the data shows that the probability distribution of slip sizes follows a power law. In addition, the frequency power spectrum follows a 1/fα pattern with α in the range 1.1–1.3. We perform a careful analysis of all the properties, beyond the two just mentioned, which are required to imply the presence of SOC. Our data strongly support the existence of SOC for stick–slip in dry sliding friction.

The Bozzolo–Ferrante–Smith (BFS) method for alloy energetics is applied to the study of ternary additions to NiAl. A description of the method and its application to alloy design is given. Two different approaches are used in the analysis of the effect of Ti additions to NiAl. First, a thorough analytical study is performed, where the energy of formation, lattice parameter and bulk modulus are calculated for a large number of possible atomic distributions of Ni, Al and Ti. Substitutional site preference schemes and formation of precipitates are thus predicted and analyzed. The second approach used consists of the determination of temperature effects on the final results, as obtained by performing a number of large-scale numerical simulations using the Monte Carlo–Metropolis procedure and BFS for the calculation of the energy at every step in the simulation. The results indicate a sharp preference of Ti for Al sites in Ni-rich NiAl alloys and the formation of ternary Heusler precipitates beyond the predicted solubility limit of 5 at. % Ti. Experimental analysis of three Ni–Al–Ti alloys confirms the theoretical predictions.

The study of surface segregation, though of great technological importance, has been largely restricted to experimental work due to limitations associated with theoretical methods. However, recent improvements in both first-principles and semiempirical methods are opening the doors to an array of new possibilities for surface scientists. We apply one of these techniques, the BFS method for alloys, which is particularly suitable for complex systems, to several aspects of the computational modeling of surfaces and segregation, including alloy surface segregation, structure and composition of alloy surfaces, and the formation of surface alloys. We conclude with the study of complex NiAl-based binary, ternary and quaternary thin films (with Ti, Cr and Cu additions to NiAl). Differences and similarities between bulk and surface compositions are discussed, illustrated by the results of Monte Carlo simulations. For some binary and ternary cases, the theoretical predictions are compared to experimental results, highlighting the accuracy and value of this developing theoretical tool.

We introduce the Bozzolo–Ferrante–Smith (BFS) method for alloys as a computationally efficient tool for aiding in the process of alloy design. An intuitive description of the BFS method is provided, followed by a formal discussion of its implementation. The method is applied to the study of the defect structure of NiAl binary alloys. The groundwork is laid for a detailed progression to higher order NiAl-based alloys linking theoretical calculations and computer simulations based on the BFS method and experimental work validating each step of the alloy design process.

Computer modelling of atomic processes on surfaces and bulk materials has seen great progress in the last decade due to the advent of semiempirical methods. In this work, we extend the scope of these techniques to deal with thin films, an area where the progress in large scale atomistic modelling lags behind similar progress in bulk alloy design and surface structure analysis. These techniques rely on solid theoretical background and provide the computational economy that allows for the treatment of complex problems. We discuss one of these techniques, the BFS (Bozzolo–Ferrante–Smith) method for alloys, and illustrate its application to the problem of thin film formation. In particular, we show modelling results on the formation of binary and ternary films of Ni and Al with either Ti or Cr alloying additions. Differences and similarities with the corresponding ternary bulk alloys are discussed. Radical changes in layer-by-layer composition are observed in both cases. While Ti additions induce the formation of a particular ternary phase close to the (110) surface, Cr additions result in the formation of an α-Cr precipitate close to the surface in addition to the changes in surface composition.